Undoped and C-doped n-type β-FeSi₂ thin films were epitaxially grown on p-type Si substrates by sputtering and their heterojunction diode performances were experimentally studied. The near-infrared photodetection, at a wavelength of 1.3 µm, in these heterojunction diodes was clearly improved as compared to the heterojunctions comprising undoped-β-FeSi₂. From X-ray diffraction and Raman spectroscopic measurements, there were no evident structural differences between the undoped and C-doped films. C-doping hardly affects the crystallization and epitaxial growth of β-FeSi₂. The enhancement in the diode performance by C-doping might be owing to C atoms terminating dangling bonds and compensating defects in β-FeSi₂ crystals.

n-Type β-FeSi₂ thin films were deposited on p-type Si(111) substrates by conventional radio frequency magnetron sputtering at substrate temperatures of 500–600 °C without post-annealing. The epitaxial growth of β-FeSi₂ on Si(111) initiates at substrate temperatures of higher than 560 °C, and it was found that the epitaxial growth is indispensable for the n-type β-FeSi₂/p-type Si heterojunctions having rectifying action.

We have fabricated Mg₂Si pn-junction photodiodes with an Au-ring electrode and a SiO₂ passivation layer by means of a lift-off photolithography process. Current–voltage (I–V) characteristics of the photodiodes showed obvious rectifying behavior at room temperature. The ideality factor of n determined from the slope of I–V characteristics was 1.76–1.92. The photodiode showed a photoresponse with threshold energy of approximately 0.6 eV under a zero-bias condition. The intensity of peak photoresponse was improved approximately three times compared with the opaque Au-circular electrode type Mg₂Si photodiode previously reported.

The hydrogenated amorphous silicon (a-Si:H) based p–i–n diode structures Al/a-Si:H(p⁺)/a-Si:H(i)/(silicides NPs/a-Si)_x/a-Si:H(i)/a-Si:H(n⁺)/ITO/glass with multiple layers (x = 8,…,15) of the embedded narrow band semiconducting nanoparticle silicide (CrSi₂, Mg₂Si, and Ca₂Si) multistructures have been grown by combining the plasma enhanced chemical vapour deposition (PECVD) and the UHV reactive deposition epitaxy (RDE). Formation of silicide nanoparticles and multistructures has been confirmed in-situ by the Auger electron spectroscopy (AES) and electron energy loss spectroscopies (EELS) and ex-situ by optical absorbance and Raman spectroscopies. The I–V curves of the a-Si:H p–i–n diodes with embedded silicide NP multistructures have shown the maximal forward current for Ca₂Si nanoparticles. The room temperature electroluminescence has been observed in the near infrared region for diodes with embedded Ca₂Si and Mg₂Si NPs multistructures.

We have investigated oxidation behaviors of nano-composite phase with β-FeSi₂ nanocrystals (β-NCs) and Si on Si substrates. IR absorption measurements revealed that only oxidation of Si into SiO₂ proceeded in the nano-composite phase. This fact is very important for realization of a novel composite phase with β-NCs and SiO₂, which may contribute to enhancement of light emission and to prevent a large thermal quenching of light emission observed in the composite phase with β-NCs and Si.

We have investigated photoluminescence (PL) behaviors of nano-composite phase of β-NCs embedded in SiO₂ (β-NCs/SiO₂). The inhomogeneous spectra consisting of the A, B, and C emission bands were observed. PL enhancement also was confirmed in comparison with β-NCs/Si. Under high pumping rate, we observed PL spectra near room temperatures (~270 K). This fact means that oxidation of the nano-composite phase can contribute to reduction of thermal quenching, which may come from increase of band offsets around β-NCs.

We have investigated PL behavior of β-FeSi₂ nanocrystals controlled by transport of holes in Cu-doped n-type Si substrates. PL enhancement was observed and PCI-PL measurements revealed that PL enhancement was attributed to a transport process of holes with a larger time constant in Cu-doped n-Si substrate in which an interval trap process is controlled by Cu doping.

We have investigated photoluminescence (PL) behaviors of the Cu-doped β-FeSi₂ thin film/Si heterostructure. Pronounced enhancement of an intrinsic A band and an impurity-related C band emissions has been observed in all the Cu-doped samples. The photo-carrier injection (PCI)-PL measurements have revealed that the PL enhancement is attributed to dynamic process of migration of holes where a repeated trap process of holes can be controlled by Cu-doping.

Effects of lattice deformations on Raman spectra were investigated in β-FeSi₂ epitaxial films grown on Si substrates. The lattice of the epitaxial film was deformed depending on thermal-annealing temperature (T_a). In Raman spectra, Raman lines of A_g-mode (iron displacements) showed a frequency shift with increasing T_a. The shift of Raman lines showed that the residual strain in the epitaxial films was changed by the lattice deformation. In the temperature dependence of Raman spectra, the temperature shifts of the Raman lines were found to be different between β-FeSi₂(100) || Si(001) and β-FeSi₂(110)(101) || Si(111) epitaxial films.

We have investigated the band structure of K₈Ga₈Si₃₈ clathrates by using optical reflectance measurements and first-principle calculations based on the density functional theory. The imaginary part of the dielectric function ε₂ was calculated from the normal reflectance spectrum via the Kramers–Kronig analysis. The experimental ε₂ values showed a fundamental absorption edge near 1 eV. From the analysis of the absorption spectrum, we found that the value of the absorption edge energy is approximately 1.2 eV for a K₈Ga₈Si₃₈ clathrate crystal.

Mg₂Si thermoelectric devices are fabricated using reused-Si by a liquid–solid phase reaction and a spark plasma sintering process. Seebeck coefficients are comparable to those for a device prepared with pure Si. However electric conductivities are lower, resulting in lower power factors. Al-doping up to 4% is found to be effective to improve the electrical conductivity.

The radio frequency plasma enhanced chemical vapor deposition (PECVD) of the hydrogenated amorphous silicon (a-Si:H) combined with the vacuum evaporation of magnesium followed by the hydrogen plasma treatment has been successfully applied for the in-situ deposition of magnesium silicide nanoparticles embedded in hydrogenated amorphous silicon thin layers. The homogeneous coverage of a-Si:H surface by magnesium silicide nanoparticles with diameter below 10 nm has been confirmed by the scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS).

In this paper we introduce amorphous hydrogenated silicon thin films (a-Si:H) deposited by PECVD with embedded magnesium silicide (Mg₂Si) nanoparticles (NPs) which are created by reactive laser ablation (RLA) of Mg target in low pressure of silane (SiH₄). Both techniques are periodically changed in short intervals–each of the monolayers of Mg₂Si NPs is covered by thin a-Si:H film. The physical characteristics of those films are studied not only on high quality optical substrates but in diode structures too. As a final result we introduce the voltage dependence of the electroluminescence of deposited diodes.

We have grown undoped n-BaSi₂ epitaxial films with different grain sizes on Si(111) and characterized their minority-carrier lifetime, τ. We found that τ value in undoped n-BaSi₂ did not depend on average grain area, but on surface condition. The samples with mirror surfaces had large τ of about 0.4 µs and those with cloudy surface small τ of about 8 µs. We tried to cap the sample surface in situ with a 3 nm Ba or Si layer in order to control the surface of BaSi₂, and succeeded to intentionally form BaSi₂ with large τ.

Al-induced layer-exchanged Ge (ALILE-Ge) combined with epitaxy is a promising way to fabricate advanced electronic optical devices on foreign substrates as well as Si large-scale integrated circuits. The presence of Ge islands on the surface of the ALILE-Ge seed layer was a problem because the islands deteriorated the crystal quality of the epitaxial layer. This paper gives a solution to this problem: the Ge islands were selectively etched by H₂O₂ treatment. The ALILE-Ge seed layer was protected by using the oxidized Al membrane, prepared between Ge and Al, as an etching mask. By initially preparing the thick Ge layer and then removing the excess Ge islands, we improved the coverage of the ALILE-Ge seed layer on the substrate. The resulting ALILE-Ge provided a high (111) orientation fraction (96%) and large grains (>100 µm). This Ge layer appears promising for use in seed layers for epitaxial Ge, group III–V compound semiconductors, and other advanced materials.

We investigated the surface potential distributions around grain boundaries (GBs) in phosphorus (P)-doped n-BaSi₂ thin-films by Kelvin probe force microscopy (KFM) and the crystal planes constituting GBs by transmission electron microscopy (TEM). By KFM measurements, it was found that the GBs in P-doped n-BaSi₂ are different from those in undoped BaSi₂; undoped n-BaSi₂ has a downward band bending around the GBs with barrier heights of approximately 30 meV. In contrast, P-doped n-BaSi₂ has an upward band bending with barrier heights of approximately 15 meV. TEM observation revealed that most of the GBs in P-doped BaSi₂ are composed of BaSi₂ (011)/() planes. This result is the same as that in undoped BaSi₂.

Organic thin film solar cells are potential “next generation” solar cells. Many p-type semiconductors have been used in organic solar cells, but there have been far fewer reports involving n-type organic semiconductors. Developing new n-type organic semiconductors is therefore desirable. Decaphenylpentasilane (DPPS) thin films were spin-coated from solutions containing boron (B), and the effects of B addition on film microstructures and electronic properties were investigated. Microstructures of DPPS thin films were investigated by X-ray diffraction, and DPPS thin films doped with B [DPPS(B)] showed the reduction of crystallinity upon annealing at 300 °C, while DPPS thin films exhibited crystalline structures. DPPS(B) thin films exhibited decreased electrical resistances upon the B doping and annealing. The desorption of phenyl and methyl groups from the DPPS(B) thin films was observed by Raman scattering measurements.

Fullerene-based photovoltaic devices with an inverted structure containing silicon 2,3-naphthalocyanine bis(trihexylsilyloxide) (SiNc) were fabricated and characterized. SiNc worked as a donor material, and showed optical absorption at ~800 nm. C₆₀ and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) were used as acceptor materials, and C₆₀ or PCBM/SiNc system solar cells showed incident photon to current conversion efficiency in the range of 700–800 nm. The PCBM/SiNc solar cell provided high open circuit voltage of 0.74 V. From the energy level diagram, the higher lowest unoccupied molecular orbital level of PCBM contributed to the high open circuit voltage. The surface roughness at the C₆₀/SiNc interface is larger compared to that at the PCBM/SiNc interface, which resulted in the high short circuit current density of the C₆₀/SiNc solar cell.

Fe₃Si/FeSi₂/Fe₃Si trilayered junctions were prepared on Si(111) by facing targets direct-current sputtering combined with a mask method, and spin valve signals in current-perpendicular-to-plane (CPP) geometry was investigated for the change of the magnetization alignment. The shape of magnetization curves evidently exhibited that an antiparallel alignment is realized owing to a difference in the coercive force between the top and bottom Fe₃Si layers. The electrical resistance was alternately changed for the formation of parallel and antiparallel alignments with the magnetic field. The spin valve signals in the Fe₃Si/FeSi₂/Fe₃Si trilayered junctions were experimentally demonstrated.

Current-perpendicular-to-plane junctions with different cross-sectional areas were fabricated from Fe₃Si/FeSi₂ artificial lattice films, wherein antiferromagnetic interlayer coupling was induced across the FeSi₂ spacer layers, by employing a focused ion beam (FIB) apparatus. Evident hysteresis loops in the electrical resistance for current injection due to current-induced magnetization switching (CIMS) were observed. The average value of critical current densities for inducing CIMS was 2 × 10² A/cm², which is at least three orders of magnitude smaller than values that have ever been reported. This might be because CIMS in our junctions is induced by the destruction of the AF interlayer coupling, which differs from the general mechanism of CIMS.

One of the most powerful and reliable methods to directly observe the switching of interlayer coupling magnetization in multilayer films is polarized neutron reflection. In this study, polarized neutron reflection was employed for artificial lattices comprising alternately accumulated ferromagnetic Fe₃Si and semiconducting FeSi₂ layers prepared by facing-targets direct-current sputtering. The switching of interlayer coupling magnetizations was directly observed, which was well consistent with that expected form the magnetization curves.

Current-perpendicular-to-plane structural Fe₃Si/FeSi₂/Fe₃Si trilayered junctions were prepared on Si(111) by facing targets direct-current sputtering combined with a mask method. The shape of magnetization curves evidently exhibited that an antiparallel alignment is realized owing to a difference in the coercive force between the top and bottom Fe₃Si layers. In addition, it was demonstrated that the antiparallel alignment in the wide range of applied magnetic field can be realized by forming a crossover structure, which is owing to an enhanced difference in the effective magnetic field between the top and bottom Fe₃Si layers aligned perpendicularly to each other.